1. Field of Invention
This invention relates to a fine insulated electric wire having low dielectric constant, and a process for producing the same.
2. Prior Art
Thin insulation layers are conventionally formed on a conductor by a foaming extrusion method as described in JP-B-57-30253 (the term "JP-B" as used herein means as "examined Japanese patent publication"). This technique generally comprises foaming polyolefinic resins with a chemical foaming agent (e.g. azodicarbonamide), an inert gas (e.g. nitrogen or argon) or a gaseous or liquid hydrocarbon or fluorocarbon, which are used either alone or as admixtures, to thereby form an insulation coating layer having low dielectric constant due to enhanced porosity.
U.S. Pat. Nos. 3,953,566 and 4,187,390 describe another method of forming an insulation layers, in which a fluorine resin tape provided with enhanced porosity by stretching is wound over a conductor. Compared to the foaming extrusion technique, this method which involves winding a tape material having a known dielectric constant over the conductor is capable of not only insuring consistency in the dielectric constant of an insulation layer but also producing a thin and high-porosity insulation layer.
Two other methods of forming insulated electric wires are proposed in JP-B-56-43564 and JP-B-57-39006. The first patent discloses a method in which microspheres or foamable spheres that have a particle size of from several microns to several millimeters and that are made of an inorganic material such as glass or alumina are coated with a thermoplastic resin and then melt-extruded The second patent discloses a method in which a thermoplastic resin such as polyethylene or polyvinyl chloride and inorganic microspheres are dissolved in a solvent such as xylene and the solution is applied onto a conductor, with the resulting coating being dried to form an insulated electric wire.
An increasing demand for fine transmission lines capable of carrying high-density signal has arisen in the medical field, computerized measurement field and various other fields, and there is a strong need for the development of fine insulated electric wires that have a thin insulation coating applied to a fine conductor and that yet have low dielectric constant.
None of the prior art methods described above are suitable for meeting this need. In the method described in JP-B-57-30253, melting of a polyolefinic resin, its foaming and coating onto a conductor are performed simultaneously by means of a screw extruder, so it is difficult to attain a high degree of foaming in thin insulation layers and the lower limit of the coating thickness that can be achieved is no smaller than 200 .mu.m. Another defect of this method is that it is not easy to control the degree of foaming.
The methods described in U.S. Pat. Nos. 3,953,566 and 4,187,390 have the inherent problem that the formation of local asperities in the surface of insulation layer is unavoidable and the linear speed of wire production is very low.
The methods described in Jp-B-56-43564 and JP-B-57-39006 are capable of controlling the degree of foaming in an easy way but they have the following defects. In the method described in JP- -56-43564 which extrusion-coats a conductor with hollow or foamable spheres having a thermoplastic resin coated thereon, the thermoplastic resin on the microspheres melts to be applied onto the conductor and is thereafter cooled to join the microspheres together. Therefore, if the thickness of the thermoplastic resin layer is reduced in order to attain high porosity, the mechanical strength, in particular, the elongation, of the insulation layer formed on the conductor will deteriorate considerably. If, on the other hand, the thickness of the thermoplastic resin layer on the surface of microspheres is increased in order to retain the mechanical strength of the insulation layer, the porosity of the insulation layer will decrease eventually to increase the dielectric constant of the electric wire. Further, in order to withstand the high temperature (.gtoreq.150.degree. C.) and pressure that develop in the extruder for a long time, the microspheres that can be used are limited to those which are made of inorganic materials such as glass and alumina. However, such microspheres intrinsically have high dielectric constant and are not suitable for the purpose of producing low-dielectric constant, low-loss cables.
The method described in JP-B-57-39006 comprises dissolving inorganic microspheres and a thermoplastic resin such as polyethylene or polyvinyl chloride in a solvent such as xylene, applying the solution onto a conductor and drying the resulting coating to form an insulated electric wire. This approach also applies heat for drying purposes in the same manner as that in JP-B-56-43564. In this case, heating temperature is relatively lower and a time required for heating process is also shorter than those in JP-B-56-43564. However, the heating time is not a moment which is shorter than 1 to 2 seconds. As a result, the microspheres that can be used are limited, which introduces difficulty in producing low-dielectric constant, low-loss cables. Further, the need for uniformly drying the applied liquid composition by evaporating the solvent renders the production speed very low.